Microscope objective lens

ABSTRACT

Apochromatic-type microscope objective lenses are disclosed having a high numeral aperture number with few component lenses. The objective lenses are operable to correct any of various aberrations arising due to use of the objective lens with cover glasses having variable thickness. The objective lenses comprise a first positive lens group comprising a positive meniscus lens with a concave surface facing objectwise, a second positive lens group comprising a positive cemented lens in which the cemented surface has negative refractive power and a negative cemented lens in which the cemented surface has positive refractive power, and a third (negative) lens group comprising at least one cemented lens. The second and third lens groups are movable on the optical axis relative to the first lens group so as to facilitate correction of aberrations arising from use of cover glasses having variable thickness.

FIELD OF THE INVENTION

This invention pertains to microscope objective lenses, especiallyapochromatic-type microscope objective lenses.

BACKGROUND OF THE INVENTION

Generally, microscope objective lenses are designed to be used with acover glass (i.e., a clear, thin, flat, parallel-plane plate placedbetween the specimen and the objective lens) of a specific thickness.However, all cover glasses have some variation in thickness, and suchvariations in thickness can affect the lens performance. The effect ofvariable cover-glass thickness on lens performance is small if thenumerical aperture (NA) of the objective lens is 0.5 or less, and can bepronounced with conventional objective lenses having a numericalaperture of greater than about 0.75.

With conventional objective lenses having a "high" numerical aperture(NA) (i.e., 0.85 or greater), be markedly deleterious, especially withrespect to the focusing performance of the lens. This problem can rendercertain such lenses unsuitable for use.

With certain "brigjt" microscope objective lenses having a highnumerical aperture, NA=0.85 or higher, a "correcting" lens group or"correcting ring" is sometimes employed to correct various aberrationsarising from variable cover-glass thickness.

Examples of microscope objective lenses including correcting rings aredisclosed, e.g., in Japanese Kokai patent document Nos. Sho 61-275812,Hei 3-50517, and Hei 5-119263.

The objective lenses disclosed in the Kokai '812 document comprises afirst lens group having a positive refractive power, a second lens groupcomprising divergent cemented surfaces (i.e., surfaces having negativerefractive power) as well as convergent cemented surfaces (i.e.,surfaces having positive refractive power), and a third lens group witha negative refractive power. The second and third lens groups comprise a"correcting" lens group that is movable along the optical axis of theobjective lens. This microscope objective lens has a numerical apertureNA=0.95, and can maintain nearly maximal brightness when used dry with acover glass having a thickness range of 0.11 to 0.23 mm. However, suchlenses have a problem with variation of chromatic difference ofspherical aberration that arises whenever the correcting lens group ismoved.

The objective lenses disclosed in the Kokai '517 document comprise, inorder from the object side, a first lens group having positiverefractive power, a second lens group having positive refractive power,and a third lens group having negative refractive power. The second lensgroup is movable along the optical axis and purportedly functions as a"correcting" lens group. Unfortunately, the second lens group iscomposed of four lens elements (three of which are cemented together toform a triplet and one simple lens element) which constitutes too manyelements for the lens group to exhibit satisfactory performance as acorrecting lens group. Also, the lens diameter is large, which tends tocause problems with aberrations (especially shifts toward the opticalaxis) arising from eccentricity.

The microscope objective lens disclosed in the Kokai '263 document has anumerical aperture NA=0.93 and includes a correcting lens groupconsisting of a cemented doublet lens. Unfortunately, however, there aretoo many lenses (thirteen) in the overall objective lens, which isprohibitively expensive.

SUMMARY OF THE INVENTION

The present invention addresses the shortcomings of the prior artdiscussed above. A key object of the invention is to provide amicroscope objective lens having a large numerical aperture and thatcorrects aberrations, especially spherical aberrations, arising fromvariability in the cover-glass thickness. Another object is to providean apochromatic-type microscope objective lens having a relatively smallnumber of lenses and a high numerical aperture.

To such end, microscope objective lenses are provided, according to theinvention, that comprise, in order from the object side, a first lensgroup G1 having a positive refractive power, a second lens group G2having a positive refracive power, and a third lens group G3 having anegative refractive force. The first lens group G1 comprises a positivemeniscus lens having a concave surface facing the object side. Thesecond lens group G2 comprises a first cemented lens L21 with positiverefractive power and including a negative lens L2n cemented to apositive lens L2p. The cemented surface has anegative refractive power.The second lens group G2 also comprises a second cemented lens L22 withnegative refractive power and having a cemented surface with positiverefractive power. The third lens group G3 has at least one cementedlens. The second lens group G2 and the third lens group G3 are axiallymovable together along the optical axis relative to the first lens groupto correct any variable aberrations, especially spherical aberrations,arising from variability in the thickness of the cover glass.

The microscope objective lens of the invention also preferably satisfiesthe following conditional expressions:

    0.33<n.sub.2n -n.sub.2p                                    ( 1)

    0.02<D.sub.12 / <1.3                                       (2)

wherein n_(2n) is the refractive index (relative to the D-line) of thenegative lens L2n; n_(2p) is the refractive index (relative to theD-line) of the positive lens L2p; D₁₂ is the axial distance (which canbe varied) in air between the first lens group G1 and the second lensgroup G2; and f is the focal length of the objective lens.

The foregoing and additional features and advantages of the presentinvention will be more readily apparent from the following detaileddescription, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical diagram showing general aspects of a microscopeobjective lens according to the invention as well as specific aspects ofExample Embodiment 1.

FIGS. 2(a)-2(c) are plots, for Example Embodiment 1, of sphericalaberration exhibited by the objective lens when used with a cover glassof 0.11, 0.17, and 0.23 mm thickness, respectively.

FIG. 3 is an optical diagram showing specific aspects of ExampleEmbodiment 2.

FIGS. 4(a)-4(c) are plots, for Example Embodiment 2, of sphericalaberration exhibited by the objective lens when used with a cover glassof 0.08, 0.17, and 0.3 mm thickness, respectively.

DETAILED DESCRIPTION

It is generally known that, in order to achieve the best flatness of theimage plane, the Petzval sum of the lens should be at or nearly at zero.Certain telephoto lenses are known that achieve a low Petzval sum usinga relatively simple configuration. Telephoto-type lenses comprising afront lens group having a positive refractive power and a rear lensgroup having a negative refractive power force generally have a smallangle of view. Telephoto-type lenses are often used withhigh-magnification lenses having a relatively small angle of view. Thisinvention exploits a telephoto-type lens system (i.e., a systemcomprising a positive lens group on the object side and a negative lensgroup on the image side) with a relatively high-magnification dryobjective lens.

A representative configuration of an objective lens according to theinvention, as shown in FIG. 1, comprises (starting on the object (left)side) a first lens group G1 having positive refractive power, a secondlens group G2 having positive refractive power, and a third lens groupG3 having negative refractive power.

The second lens group G2 comprises a first cemented lens L21 havingpositive refractive power and including a negative lens L2n cemented toa positive lens L2p. The first cemented lens L21 has a cemented surface11 having a strongly negative refractive power. (The power is negativeeven though the cemented surface has a positive curvature radius becausen_(L21>n) _(L22).) The second lens group G2 also comprises a secondcemented lens L22 having negative refractive power. The second cementedlens L22 has a cemented surface 14 having a positive refractive power.

The second lens group G2 and the third lens group G3 are regarded as"corrective" lens groups. Within the second and third lens groups, thecemented surface formed by cementing the lens L2n to L2p is termed the"F surface" which is a term used to denote a cemented surface having astrongly negative refractive power. Generally, by adjusting the heightof an entering ray in relation to the F surface, variations in sphericalaberration arising from variability in the thickness of the cover glassare corrected.

As indicated above, the F surface is located in a corrective lens group.By moving the corrective lens group along the optical axis, the Fsurface is also moved. Thus, the height of the entering ray can beadjusted in relation to the F surface. This, in turn, permits variousaberrations (especially spherical aberrations) arising from variabilityin the thickness of the cover glass to be corrected.

Spherical aberration is proportional to the height (i.e., lateraldisplacement from the optical axis) of the ray. The present inventionallows a reduction in the amount of axial distance the corrective lensgroup has to move in order to increase the available diameter of the Fsurface.

Objective lenses according to the invention also satisfy the followingconditional expressions:

    0.33<n.sub.2n -n.sub.2p                                    (1)

    0.02<<D.sub.12 /f<1.3                                      (2)

wherein n_(2n) is the refractive index (relative to the D-line) of thenegative lens L2n, n_(2p) is the refractive index (relative to theD-line) of the positive lens L2p, D₁₂ is the axial distance, in air,between the first lens group G1 and the second lens group G2, and f isthe focal length for the entire objective lens system.

The distance D₁₂ between the first lens group G1 and the second lensgroup G2 refers to the distance, along the optical axis between thesurface in the first lens group G1 that is closest to the image side andthe surface in the second lens group G2 that is closest to the objectside.

Conditional expression (1) specifies, within the second lens group G2,an optimal range for the refractive index difference between thenegative lens L_(2n) and the positive lens L_(2p). These two lenses arecemented together to form the F surface. Falling below the lower limitof conditional expression (1), causes the refractive-index difference tobe too small. Thus, in order to achieve satisfactory correction ofspherical aberration, the curvature of the cemented F surface would haveto be increased, resulting in unacceptable chromatic aberration.

Conditional expression (2) relates to the movement range of thecorrective lens group (i.e., the second and third lens groups) andspecifies an optimal range for the axial distance between the first andsecond lens groups. Exceeding the upper limit of conditional expression(2) would cause the axial distance between the first and second lensgroups to be too great and the balance of lateral chromatic aberrationwould be lost. (Moving the corrective lens group causes a change inlateral chromatic aberration. Also, if the first and second lens groupsbecome too close together, manufacturing problems arise.

It is desirable to correct the individual lateral chromatic aberrationof the first and second lens groups as much as possible.

More specifically with each of the lens groups, the first lens group G1includes at least one positive meniscus lens having a concave surfacefacing the object side. The second lens group G2 includes first andsecond cemented lenses L21, L22, respectively, as described above. Thethird lens group G3 includes at least one cemented lens.

Each of the example embodiments described below was evaluated using, onthe image side of the objective lens and along the optical axis, afocusing lens. The axial air space between the objective lens and thefocusing lens was 150 mm.

Also, the various aberration plots for each example embodiment weregenerated with the combination of the respective objective lens and thefocusing lens as discussed above. However, even if the air spacedistance were to change slightly from 150 mm, it has been verified thatalmost no change arises in aberrations.

The focusing lens used to evaluate the objective lenses comprises, inorder from the object side, a cemented positive lens including abiconvex lens cemented to a biconcave lens, and a cemented positive lensincluding a biconvex lens cemented to a biconcave lens.

Table 1, below, provides data concerning the focusing lens. In Table 1,the left-hand column pertains to surface numbers in order from theobject side. The second column, designated "r", pertains to thecurvature radius of each lens surface. The third column, designated "d",pertains to the axial distance between adjacent lens surfaces. Thefourth column, designated "n_(d) ", pertains to the refractive index(relative to the D-line; λ=587.63 nm) of each of the lens elements. Theright-hand column, designated "v_(d) ", pertains to the Abbe number ofeach of the lens elements.

                  TABLE 1                                                         ______________________________________                                        Surface                                                                       No.       r (mm)   d (mm)     n.sub.d                                                                             ν.sub.d                                ______________________________________                                        21        75.0450  5.1        1.6228                                                                              57.03                                     22        -75.045  2.0        1.7500                                                                              35.20                                     23        1600.5800                                                                              7.5                                                        24        50.2560  5.1        1.6676                                                                              42.00                                     25        -84.5410 1.8        1.6127                                                                              44.41                                     26        36.9110                                                             ______________________________________                                    

EXAMPLE EMBODIMENT 1

FIG. 1 depicts specific features of an objective lens according to thisexample embodiment.

The first lens group G1 comprises, in order from the object side, apositive meniscus lens (surfaces 1-2) having a concave surface 1 facingthe object side, a first positive meniscus lens (surfaces 3-4) with theconcave surface 3 facing the object side, a second positive meniscuslens (surfaces 5-6) having a concave surface 5 facing the object side,and a cemented positive lens (surfaces 7-9) including a negativemeniscus lens having a convex surface facing the object side and abiconvex lens.

In order from the object side, the second lens group G2 comprises apositive cemented lens L21 including a negative meniscus lens L2n havinga convex surface 10 facing the object side and a biconvex lens, and anegative cemented lens L22 including a biconvex lens (surfaces 13-14)and a biconcave lens (surfaces 14-15). The cemented surface 11 of thepositive cemented lens L21 has a negative refractive power and thecemented surface 14 of the negative cemented lens L22 has a positiverefractive power.

Furthermore, in order from the object side, the third lens group G3comprises a negative cemented lens including a biconcave lens (surfaces16-17) and a biconvex lens (surfaces 17-18).

Table 2 presents optical data concerning this first example embodiment.In Table 2, f is the focal length of the microscope objective lens (inmm), NA is the numerical aperture, β is the magnification of a systemcomprising objective lens and the focusing lens of Table 1, and WD isthe working distance (in mm).

In Table 2, the left-hand column pertains to surface numbers in orderfrom the object side. The second column, designated "r", pertains to thecurvature radius of each lens surface. The third column, designated "d",pertains to the axial distance between adjacent lens surfaces. Thefourth column, designated "n_(d) ", pertains to the refractive index(relative to the D-line; λ=587.63 nm) of each of the lens elements. Theright-hand column, designated "v_(d) ", pertains to the Abbe number ofeach of the lens elements.

                  TABLE 2                                                         ______________________________________                                                    f = 5.0 mm                                                                    NA = 0.95                                                                     β = 40x                                                                  WD = 0.14 mm                                                      ______________________________________                                        Surface                                                                       No.       r (mm)   d (mm)     n.sub.d                                                                             ν.sub.d                                ______________________________________                                        1         -2.1418  4.30       1.6935                                                                              53.76                                     2         -3.7147  0.10                                                       3         -15.3747 3.30       1.4978                                                                              82.52                                     4         -7.6264  0.15                                                       5         -34.5141 3.75       1.4978                                                                              82.52                                     6         -12.3011 0.10                                                       7         34.7490  1.15       1.6889                                                                              31.08                                     8         12.2878  7.50       1.4978                                                                              95.57                                     9         -12.2878 (d.sub.9 = D.sub.12 = variable)                            10        38.3740  1.15       1.8052                                                                              25.35                                     11        8.4327   6.80       1.4339                                                                              95.57                                     12        -33.7370 1.80                                                       13        203.9400 5.50       1.8052                                                                              25.41                                     14        -10.9804 1.60       1.4339                                                                              95.57                                     15        124.9710 6.60                                                       16        -27.1710 1.00       1.7668                                                                              46.80                                     17        10.4119  3.00       1.5481                                                                              45.87                                     18        -60.8420 150                                                        ______________________________________                                        Values of D.sub.12 For Correction of Aberrations                              Cover Glass Thickness                                                                         Variable Distance D.sub.12                                    ______________________________________                                        0.11 mm          0.65 mm                                                      0.17 mm         1.785 mm                                                      0.23 mm          3.19 mm                                                      ______________________________________                                        Values of Conditional Expressions                                             ______________________________________                                                  (1) n.sub.2n - n.sub.2p = 0.37                                                (2) D.sub.12 /f = 0.13 ˜ 0.638                                ______________________________________                                    

FIGS. 2(a)-2(c) present spherical aberration plots for the first exampleembodiment when used with a cover glass of the stated thickness. FIG.2(a) is a plot for a cover-glass thickness of 0.11 mm. FIG. 2(b) is aplot for a cover-glass thickness of 0.17 mm. FIG. 2(c) is a plot for acover-glass thickness of 0.23 mm.

In FIGS. 2(a)-2(c), NA is the numerical aperture, d is the D-line(λ=587.63 nm), C is the C-line (λ=656.3 nm) and F is the F-line (λ=486.1nm).

As is clear from each of the spherical aberration plots for this exampleembodiment, spherical aberrations arising from changes in the thicknessof the cover glass are very well corrected.

EXAMPLE EMBODIMENT 2

FIG. 3 depicts specific features of an objective lens according to thisexample embodiment.

The first lens group G1 comprises, in order from the object side, apositive meniscus lens (surfaces 1-2) having a concave surface 1 facingthe object side, a first positive menicus lens (surfaces 3-4) with theconcave surface 3 facing the object side, a second positive meniscuslens (surfaces 5-6) having a concave surface 5 facing the object side,and a cemented positive lens (surfaces 7-9) including a negativemeniscus lens having a convex surface facing the object side and abiconvex lens.

In order from the object side, the second lens group G2 comprises apositive cemented lens L21 including a negative meniscus lens L2n havinga convex surface 10 facing the object side and a biconvex lens, and anegative cemented lens L22 including a biconvex lens (surfaces 13-14)and a biconcave lens (surfaces 14-15). The cemented surface 11 of thepositive cemented lens L21 has a negative refractive power and thecemented surface 14 of the negative cemented lens L22 has a positiverefractive power.

Furthermore, in order from the object side, the third lens group G3comprises a negative cemented lens including a biconvex lens (surfaces16-17) and a biconcave lens (surfaces 17-18).

Table 3 presents optical data concerning this second example embodiment.In Table 3, f is the focal length of the microscope objective lens (inmm), NA is the numerical aperture, β is the magnification of a systemcomprising objective lens and the focusing lens of Table 1, and WD isthe working distance (in mm).

In Table 3, the left-hand column pertains to surface numbers in orderfrom the object side. The second column, designated "r", pertains to thecurvature radius of each lens surface. The third column, designated "d",pertains to the axial distance between adjacent lens surfaces. Thefourth column, designated "n_(d) ", pertains to the refractive index(relative to the D-line; λ=587.63 nm) of each of the lens elements. Theright-hand column, designated "v_(d) ", pertains to the Abbe number ofeach of the lens elements.

                  TABLE 3                                                         ______________________________________                                                    f = 5.0 mm                                                                    NA = 0.9                                                                      β = 40x                                                                  WD = 0.14 mm                                                      ______________________________________                                        Surface                                                                       No.       r (mm)   d (mm)     n.sub.d                                                                             ν.sub.d                                ______________________________________                                        1         -2.1461  4.25       1.6935                                                                              53.76                                     2         -3.7862  0.10                                                       3         -15.7896 3.30       1.4978                                                                              82.52                                     4         -7.6264  0.15                                                       5         -38.7484 3.75       1.4978                                                                              82.52                                     6         -12.2561 0.10                                                       7         34.7490  1.20       1.6889                                                                              31.08                                     8         12.2881  7.50       1.4978                                                                              95.57                                     9         -12.2933 (d.sub.9 = D.sub.12 = variable)                            10        43.1326  1.15       1.8052                                                                              25.35                                     11        8.4532   6.80       1.4339                                                                              95.57                                     12        -32.0003 1.80                                                       13        -411.5836                                                                              5.50       1.8052                                                                              25.41                                     14        -10.5541 1.60       1.4339                                                                              95.57                                     15        132.4293 6.60                                                       16        41.9894  3.00       1.5481                                                                              45.87                                     17        10.7049  1.00       1.7668                                                                              46.80                                     18        29.1202  150                                                        ______________________________________                                        Values of D.sub.12 For Correction of Aberrations                              Cover Glass Thickness                                                                         Variable Distance D.sub.12                                    ______________________________________                                        0.08 mm         0.39 mm                                                       0.17 mm         1.89 mm                                                        0.3 mm          4.8 mm                                                       ______________________________________                                        Values of Conditional Expressions                                             ______________________________________                                                  (1) n.sub.2n - n.sub.2p = 0.37                                                (2) D.sub.12 /f = 0.78 ˜ 0.96                                 ______________________________________                                    

FIGS. 4(a)-4(c) present spherical aberration plots for the secondexample embodiment when used with a cover glass of the stated thickness.FIG. 4(a) is a plot for a cover-glass thickness of 0.08 mm. FIG. 4(b) isa plot for a cover-glass thickness of 0.17 mm. FIG. 4(c) is a plot for acover-glass thickness of 0.3 mm.

In FIGS. 4(a)-4(c), NA is the numerical aperture, d is the D-line(λ=587.63 nm), C is the C-line (λ=656.3 nm) and F is the F-line (λ=486.1nm).

As is clear from each of the spherical aberration plots for this exampleembodiment, spherical aberrations arising from changes in the thicknessof the cover glass are very well corrected.

As described above, with a magnification of about 40× and largenumerical aperture, microscope objective lenses according to thisinvention can provide excellent correction of various aberrations(especially spherical aberration) caused by variability in cover-glassthickness. Thus, the invention provides apochromatic microscopeobjective lenses that comprise few component lenses.

Whereas the invention has been described in connection with severalexample embodiments, it will be Understood that the invention is notlimited to those example embodiments. On the contrary, the invention isintended to encompass all alternatives, modifications, and equivalentsas may be included within the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A microscope objective lens comprising, on anoptical axis from the object side to the image side:(a) a first lensgroup (G1) having a positive refractive power, a second lens group (G2)having a positive refractive power, and a third lens group (G3) having anegative refractive power; (b) the first lens group (G1) comprisingfirst, second, and third positive meniscus lenses each having a concavesurface facing the object side; (c) the second lens group (G2)comprising a first cemented lens (L21) including a negative lens (L2n)cemented to a positive lens (L2p) and a cemented surface having anegative refractive power, and a second cemented lens (L22) including acemented surface having a positive refractive power; (d) the third lensgroup (G3) comprising a cemented lens consisting of a biconcave lens anda biconvex lens; (e) the second and third lens groups being movablealong the optical axis relative to the first lens group so as to correctaberrations arising as a result of using the objective lens with a coverglass having variable thickness; and (f) the lens satisfying thefollowing conditional expressions:

    0.33<n.sub.2n -n.sub.2p

    0.02<(D.sub.12)/f<1.3

wherein n_(2n) is the refractive index (relative to the D-line) of thenegative lens (L2n), n_(2p) is the refractive index (relative to theD-line) of the positive lens (L2p), (D₁₂) is the axial air space betweenthe first lens group (G1) and the second lens group (G2), and f is thefocal length of the objective lens.
 2. The microscope objective lens ofclaim 1, having a mangification of 40×.
 3. The microscope objective lensof claim 1, further having the following characteristics:(a) a focallength f=5.0 mm (b) a numerical aperture NA=0.95 (c) a magnificationβ=40× (d) a working distance WD=0.14 mm (e) surfaces, curvature radii(r), inter-surface axial distances (d), indices of refraction (n_(d)),and Abbe numbers (v_(d)), as follows:

    ______________________________________                                        Surface No.                                                                              r (mm)   d (mm)     n.sub.d                                                                             ν.sub.d                               ______________________________________                                        1          -2.1418  4.30       1.6935                                                                              53.76                                    2          -3.7147  0.10                                                      3          -15.3747 3.30       1.4978                                                                              82.52                                    4          -7.6264  0.15                                                      5          -34.5141 3.75       1.4978                                                                              82.52                                    6          -12.3011 0.10                                                      7          34.7490  1.15       1.6889                                                                              31.08                                    8          12.2878  7.50       1.4978                                                                              95.57                                    9          -12.2878 (d.sub.9 = D.sub.12 = variable)                           10         38.3740  1.15       1.8052                                                                              25.35                                    11         8.4327   6.80       1.4339                                                                              95.57                                    12         -33.7370 1.80                                                      13         203.9400 5.50       1.8052                                                                              25.41                                    14         -10.9804 1.60       1.4339                                                                              95.57                                    15         124.9710 6.60                                                      16         -27.1710 1.00       1.7668                                                                              46.80                                    17         10.4119  3.00       1.5481                                                                              45.87                                    18         -60.8420 150                                                       ______________________________________                                    


4. The microscope objective lens of claim 1, wherein the first lensgroup (G1) further comprises a cemented lens including a negativemeniscus lens cemented to a biconvex lens, the cemented lens beingsituated imagewise of the first, second, and third positive meniscuslenses.
 5. The microscope objective lens of claim 1, exhibiting anumerical aperature of at least 0.85.
 6. The microscope objective lensof claim 1, wherein the first lens group (G1) further comprises acemented lens including a negative meniscus lens cemented to a biconvexlens, the cemented lens being situated imagewise of the first positivemeniscus lens.
 7. The microscope objective lens of claim 1, wherein thesecond cemented lens (L22) in the second lens group (G2) comprises abiconvex lens cemented to a biconcave lens.
 8. The microscope objectivelens of claim 1, further having the following characteristics:(a) afocal length f=5.0 mm (b) a numerical aperture NA=0.9 (c) amagnification β=40× (d) a working distance WD=0.14 mm (e) surfaces,curvature radii (r), inter-surface axial distances (d), indices ofrefraction (n_(d)), and Abbe numbers (v_(d)), as follows:

    ______________________________________                                        Surface No.                                                                              r (mm)   d (mm)     n.sub.d                                                                             ν.sub.d                               ______________________________________                                        1          -2.1461  4.25       1.6935                                                                              53.76                                    2          -3.7862  0.10                                                      3          -15.7896 3.30       1.4978                                                                              82.52                                    4          -7.6264  0.15                                                      5          -38.7484 3.75       1.4978                                                                              82.52                                    6          -12.2561 0.10                                                      7          34.7490  1.20       1.6889                                                                              31.08                                    8          12.2881  7.50       1.4978                                                                              95.57                                    9          -12.2933 (d.sub.9 = D.sub.12 = variable)                           10         43.1326  1.15       1.8052                                                                              25.35                                    11         8.4532   6.80       1.4339                                                                              95.57                                    12         -32.0003 1.80                                                      13         -411.5836                                                                              5.50       1.8052                                                                              25.41                                    14         -10.5541 1.60       1.4339                                                                              95.57                                    15         132.4293 6.60                                                      16         41.9894  3.00       1.5481                                                                              45.87                                    17         10.7049  1.00       1.7668                                                                              46.80                                    18         29.1202  150                                                       ______________________________________                                    